Method for treatment of fluid-bearing formations

ABSTRACT

By the process of this invention, underground formations can be fractured through a series of steps comprising first building up pressure in a confined zone with a slow burning propellant sufficient to initiate the fracture. Thereafter, fast burning propellant present in the total charge is ignited to supply large volumes of gas at a pressure sufficient to extend the initiated fracture.

United States Patent [72] Inventors Clarence R. Fast;

Morton A. Mallinger, both of Tuba, Okla; Donald J. OConnor, Seymour,Ind.

Greene 3,002,559 10/1961 Hanes..... 166/63 3,270,668 9/1966 Silver166/299 X 3,318,395 5/1967 Messmer 166/63 X 3,422,760 1/1969 Mohaupt102/216 OTHER REFERENCES Frick, Thos. C., Petroleum Production Handbook,N.Y., McGraw-Hill 1962, page 47- 16 Smith, E. T. 8., Solid PropellentRocket Motors, 1958, pp. 207, 208 relied on.

Primary Examiner-Jan A. Calvert Attorneys- Paul F. l-lawley and ArthurMcllroy ABSTRACT: By the process of this invention, undergroundformations can be fractured through a series of steps comprising firstbuilding up pressure in a confined zone with a slow burning propellantsufficient to initiate the fracture. Thereafter, fast burning propellantpresent in the total charge is ignited to supply large volumes of gas ata pressure sufficient to extend the initiated fracture.

PATENTEU [E628 |97l SHEET 1 [)F 3 DONALD J. OCONNOR CLARENCE R. FASTMORTON A. MALLINGER INVENTORS ATTORNEY PATENTEDUEBZBIBTI 3,630,284

SHEET 2 [1F 3 W DONALD J. OCONNOR I 7 l W l CLARENCE R. FAST M46 M44 46MORTON AMALLINGER l x K INVENTORS.

ATTORNEX PATENTED DEB28 I97! SHEET 3 OF 3 lllllllllllllll'llll'lllllillllllllllllllllll RTIRS OSER NAG N N O Um LV OEAW. JA L w R AA NL OD R a ATTORNEY METHOD FOR TREATMENT OF FLUID-BEARING FORMATIONSINTRODUCTION The present invention relates to the use of propellants forthe purpose of stimulating the production of fluids from certainunderground formations. More particularly, it is concerned with animproved technique for using propellants under controlled conditionswhereby maximum stimulation of such formations can be obtained withoutdamage to the casing or excessive shattering of the formation as wasfrequently experienced with the use of nitroglycerine and similarmaterials. Propellants may be classified as low explosives and differfrom high explosives in that the rate of energy released byautocombustion can be controlled within limits.

BACKGROUND OF THE INVENTION In fracturing a fluid-bearing formation byuse of a propellant or other high-pressure gas releasing means, careshould be exercised so that the energy released is not so rapid as toapproach detonation rates. If so, the effect on the formation is similarto that obtained with a high explosive such as nitroglycerine. Whilewith the latter type of material a substantial portion of the formationaround the well bore is shattered, the fractures formed usually do notextend very far away from the well and frequently have a rather smalldrainage area and therefore the increase in well productivity is not asgreat as desired.

In the previous use of propellants for the purpose of well stimulation,precautions were not taken to tailor the propellant charge to theformation conditions. For example, when burning a propellant charge in awell that had previously been fractured, the burning rate was much tooslow. In some instances, the burning rate was so slow that no pressurebuilt up during the burning operation because the gaseous products ofcombustion leaked off to the fracture as fast as they were generated. Ina procedure of this kind the propellant is burned in a packed off zoneso that fracturing pressure can be produced. In low-pressure wells thepropellant charge burns at a relatively lower rate and as the pressurebuilds up under the packer it burns more rapidly but does not reach ashigh a burning rate as desired because of the character or configuration(burning area) of the charge.

BRIEF DESCRIPTION OF THE INVENTION We have now perfected a reliabletechnique for fracturing a fluid-bearing formation penetrated by a wellinvolving the use of a wire line operated casing packer and gasgenerating assembly. Briefly, we employ a propellant configuration thatwill permit slow burning of approximately one-tenth to one-fourth of thecharge in the packed ofi' zone. At this stage, enough combustionproducts are produced to increase pressure so that a fracture orfractures in the formation are initiated. The remainder of thepropellant charge should be of such a character or configuration that itburns rapidly, e.g., 0.2 in./sec. under well conditions. Rapid burningand generation of combustion gases at high pressure thus permitsextension of the fractures started by the slow burning propellant.Regulated burning of the propellant charge is accomplished by preparingthe latter so that the initial portion of the charge, e.g., 20-50 poundsof a 200-pound charge, burns from the end in he same manner as acigarette. To do this the propellant is placed in a suitable restrictormaterial such as a cardboard or waterproof paper cylinder that willresist ignition from hot gases along the sides of the propellantcontained therein. The remaining portion of the propellant is eithernaturally a faster burning material or has a configuration such that itwill ignite from the initial portion but will not burn from end to endas is true in the case of a cigarette. The aforesaid remaining portionis made up of several sustainer units which may have one or more holesin or near the center of the units so that after the initial igniterunit has burned, the remainder of the charges will burn very rapidlybecauseof their increased surface area.

It may be desirable to have the major or fast burning portion of thepropellant in sections so that it can be readily placed in the well. Inany event, these sectional joints should be resistant to the penetrationof well fluids and be restricted to the penetration of hot gases so thatwell fluids will not enter the centrally drilled hole in the propellantand prevent rapid combustion. The propellant charge may be made up of asolid igniter and, if desired, additional slow burning, e.g., 0.05 to0.1 inch/second, sustainer units. The other portionof the charge hassome fast burning sustainer units that can ignite from the slowerburning propellant sections. The fast burning units .can be prepared byembedding fast burning igniter cord in the propellant, such as, forexample, DuPonts Pyrocore igniter cord. This product is in the form of asmall diameter continuous tubing containing an ignition composition. ltscore is designed to promote ignition at the speed of detonation. Byembedding such a material in the sustainer unit, instant ignition of theunit can be obtained. When the igniter cord is ac-' tivated by theadjacent igniter or sustainer unit, the fast burning s'ustainer unit canbe made to burn at as many surfaces as desired.

In setting the propellant charge either in a cased or open hole, it isgenerally preferred to place the charge in a relatively confined,packed-off section of the well. Thus, if the bottom of the charge isplaced close, i.e., 5-10 feet from the bottom of the well, only an upperpacker need be set. However, if the zone to be fractured is at anintermediate level it should be in a confined space formed by an upperpacker, with the remainder of the hole being plugged back with sand, abridge plug or other suitable means, up to a level of from about 5 or 10feet from the base of the propellant charge. This is primarily to insureuniform and rapid burning of the major portion of the charge. If thelatter is not placed near the bottom of the well, or the spaceimmediately below the charge is not packed off or plugged back, the hotgaseous combustion products from the igniter section can travel down thewell instead of moving upwardly past the outer surfaces of the sustainerunits. This results in slower and nonuniform burning of the main charge.With relatively slow generation of the gases, fracturing extendingpressures generally cannot be reached before such gases are dissipatedout into the formation.

The composition of the propellants used in the process of our inventionvary rather widely, the principal objective being to employ a chargehaving the burning characteristics discussed above. Typical of suchpropellants are the ammonium nitrate type, preferably with from about5-10 weight percent of an asphalt binder. Usually these compositionscontain -l weight percent of a stabilizer such as toluene diamine and 5or 10 weight percent of a catalyst such as Prussian blue, am-

monium chromate, potassium chromate or a mixture of chromates with metaloxides or silicates.

Propellants of the type described are used in the form of cylindricalsections typically 4 inches in diameter and 36 inches long for theigniter units, and the sustainer units are preferably l8 inches inlength and 4 inches in diameter. Approximately 8 pounds of propellantshould be used for each linear foot of hole exposed to treatment, with aminimum of 200 pounds in the total charge for each treatment. With thissize of propellant charge, about 150,000 B.t.u. of heat and 3,600 s.c.f.of gas are generated in less than one minute. A charge of this weight(200 pounds) 4 inches in diameter is about 25 feet long. The propellantcharge is preferably carried in a polyvinyl chloride tube, closed at thebottom end. The assembly of propellant charge and carrier is affixed toa Wire line operated casing packer-hydraulic hold down tool and loweredinto the well to the desired depth. Propellant charges of the typementioned above, when used in accordance with our invention, createfractures in the formation for a distance of 50 feet or more, and willheat the areaimmediately adjacent the well to cause removal of scale andspalling of the formation face in open hole wells.

BRIEF DESCRIPTION OF THE DRAWINGS AND A PREFERRED EMBODIMENT OF THEINVENTION For a better understanding of our invention, reference is madeto the following drawings wherein:

FIG. 1 is an elevational view of apparatus used in practicing ourinvention shown as disposed in a well;

FIG. 2 is an enlargement of the lower section of the assembly shown inFIG. 1, illustrating the wiring arrangement used to activate the flamesquib which rests in the lowermost ignition unit;

FIG. 3 illustrates the apparatus of FIG. 1 after the packer andhydraulic anchor have been set, with the initial and extending fracturesformed;

FIG. 4 is a sectional elevational view of a slow burning igniter unitequipped with an electric flame squib and enclosed in a plastic casing;

FIG. 5 is a sectional view of a fast burning sustainer unit equippedwith igniter tubes;

FIG. 6 is a cross-sectional view of FIG. 5 taken along line 5-5; and

FIG. 7 is a sectional elevational view of another type of fast burningsustainer unit in which channels have been drilled into the propellantto increase the burning surface area.

Referring again to FIG. 1, the tool 2 is suspended on an armored cable 4into cased well 6. The tool is lowered into the well by means of cable 4affixed to cable head 8. Firing unit 10 containing an igniter assemblyjoins cable head 8 and anchor actuating propellant chamber 12. Thelatter contains a propellant charge varying in size depending on thedepth at which the anchor and packer are to be set. To the base ofpropellant chamber 12 the upper end of anchor 14 is engaged. Anchor I4is equipped with slips 16 that project out horizontally against thecasing (see FIG. 3) by means of gas generated when the propellant inchamber 12 is ignited. This same charge also serves to set packer 18through the upward movement of setting wedge 20. At the base of settingwedge 20 is an adapter sub 22 which joins wedge 20 with propellantcontainer 24 constructed preferably of a combustible material such asplastic, for example, polyvinyl chloride or magnesium. Within container24 is a series of propellant canisters 26 encased in a water resistantpaper 28 (see FIGS. 2 and 7).

Conductor wires 30 and 32 lead to armored cable 8 and function to fireboth the anchor actuating propellant and the main propellant charge, thelowermost end of which contains a flame squib 34 as shown in greaterdetail in FIG. 4. A suitable squib for this purpose is the typedescribed and claimed in copending application U.S. Ser. No. 25,063,filed Apr. 2, 1970 by Clarence R. Fast et al. Electrical switch 36 isclosed at ground level to introduce current from power source 38 tofiring unit 10 which in turn activates the charge in anchor actuatingpropellant chamber 12. Simultaneously, current is introduced into flamesquib 34 which serves to initiate slow burning unit 40. The anchoractivating propellant is fast acting, while the propellant in canister40 is slow burning. This permits anchor 14 and packer 18 to be set inposition before propellant charge 40 burns. The latter builds upsufficient pressure in confined space 42 (see FIG 3) to initiate afracture 44 in the formation. Pressure built up within confined zone 42by ignition of fast burning propellant 48 is sufficient to extend theinitiated fractures to a position such as that shown by dashed lines 46.If the pressure under packer l8 builds up to a higher value than thattrapped in the packer, a check valve arrangement such as that describedand claimed in copending application, U.S. Ser. No. 25,125, filed Apr.2, 1970 by Clarence R. Fast et al. may be employed. This valve islocated in adapter sub 22 and opens when excessive pressure builds upunderneath the packer, permitting the pressure to enter thepacker-anchor combination and keep the tool in a set position. Aftercompletion of the burning of the fast acting propellant, the pressure inthe tool is bled off by pulling on armored cable 4 which opens a springloaded pressure release valve. When pressure above and below packer 18is equalized, anchor slips 16 are retracted by means of built in springsand the assembly brought to the surface.

In FIG. 2, wires 30 and 32 are brought outside of the upper portion ofadapter sub 22 through an opening 50. These wires run between paperwrapping 28 and plastic carrier 24 until they reach the lowermostcanister 40 which contains the slow burning propellant. Near the base ofcanister 40, wires 30 and 32 enter a recess 52 where they make contactwith flame squib 34, as shown in FIG. 4.

FIGS. 5 and 6 illustrate an embodiment of sustainer unit 48 capable ofrapid burning and building up pressure in a confined zone sufficient toextend a formation fracture after the latter has been initiated by thecombustion of a slow burning propellant charge such as that referred toin FIGS. 2 and 4. Sustainer unit 48, as shown in FIG. 5, is equippedwith a plurality of igniter tubes 54, the number of such tubes in eachsustainer unit depending upon the speed with which it is desired for theburning process to take place.

FIG. 7 represents still another kind of rapid burning sustainer unitsuitable for building up pressure rapidly and extending the initiatedfracture. In this type of unit, the propellant 29 may be essentially ofthe same compositions as that used for the slow burning charge. However,the former is made more rapid burning by drilling holes or otherwiseforming one or more channels running lengthwise through the charge so asto increase the burning surface to provide a greater surface area perunit volume than the slow burning propellant and thereby increase thepropellants burning rate.

The process of our invention will be further illustrated by thefollowing specific examples.

EXAMPLE 1 A well in the Northwest Mallet Unit located in the SlaughterField, West Texas, was producing from 41 feet of open hole and from 40perforations in 5% inch casing from a depth of 49 l 5 to 4935 feet. Inthis case it was proposed to treat only the perforated section of thewell since production from the open hole portion was negligible. Theproduction tubing was pulled, the well plugged back with sand and cappedwith 13 feet of gypsum cement (Calseal). The tool described above wasloaded with 200 pounds of catalyzed ammonium nitrate propellant, makinga column about 4 inches in diameter and 25 feet long, the lowermost 3feet of which were slow burning. When the tool had been lowered tofiring depth, the packer and anchor were set at 4905 feet and the bottomof the propellant column was at 4930 feet, about 5 feet above the bottomperforation and 9 feet above the plugged back depth. The fluid level inthe well was 2974 feet.

The squib was fired and about 15 minutes thereafter the tool wasreleased and removed from the well. A sand pump was run into the welland placed in operation but only about one double handful of Calseal wasrecovered. No propellant or debris was found, indicating completeburning of the 200 pounds of propellant charge. Post treatmentproduction tests taken with the sand plug in place showed a substantialincrease in production. After the plug was removed, fluid recoveryincreased for several days, after which there was a gradual decline. Thetable below shows production data obtained after treatment of the well.

EXAMPLE 2 The well employed in this test was located in the Turtle BayField, Texas, and was making 20-30 barrels of oil once each week bymeans of gas lift. Before treatment the well was standing full of water.The zone of interest in this well was the Lotz sand which was to betreated through twenty perforations from 6515-6525 feet. The tool wasloaded with 250 pounds of catalyzed ammonium nitrate propellant, andwhen the packer was at a depth of 6498 feet it and the anchor were setby activation of the firing unit. Simultaneously, the flame squib in thebase of the propellant column was initiated which after a 541 seconddelay fired the ignition unit making up the lower 3 feet of propellantcharge, the latter being approximately 3 feet off bottom. By lifting thewire line on which the tool was lowered into the well, it was unset andremoved. Thereafter the well was placed back on production and producedat an average rate of 42 to 49 barrels of oil per week with negligiblewater.

The procedure described herein is applicable in oil and gas producingareas where formation damage or scale problems exist. This stimulationprocess through the application of heat and high-pressure gas to eitheropen hole or casing perforations will result in breaking through anydamaged zone or scale that may exist in perforations or in the formationaround the well. This technique is also applicable in water injectionwells to increase injectivity at selected intervals. The method shouldbe useful in producing wells where short fractures are desired toincrease productivity, particularly in wells damaged during drilling andcompletion by mud or cement filtrate and, accordingly, is to beconsidered as a well completion procedure.

We claim:

1. In a process for placing a fracture in an underground formationpenetrated by a well by the use of a generally cylindrical elongatedpropellant charge, the improvement which comprises:

placing said charge in a confined zone opposite the portion of saidformation to be fractured, said charge having a separate minor amount ofa slow burning propellant and the remainder of said charge consistingessentially of fast burning propellant, said slow and fast burningpropellants being in an abutting relationship,

igniting said slow burning propellant to build up a resultant gaspressure sufficient to initiate a fracture in said formation; and

thereafter igniting said fast burning propellant in said zone by use ofheat generated from the combustion of said slow burning propellantwhereby the gas pressure thus formed is adequate to extend said fracturea still further distance from said well.

2. The method of claim 1 wherein the slow and fast burning propellantsare the same composition but the portion thereof constituting the fastburning material has a substantially greater surface area per unitvolume than the slow burning propellant.

3. The method of claim 1 wherein said well is a water injection well andwherein the formation in the immediate vicinity of said well andopposite said confined zone has decreased substantially in its abilityto take water at normal injection pressures.

4. In a process for stimulating the flow of fluid from an undergroundformation penetrated by a well, the improvement which comprises:

placing in a confined zone opposite said formation an elongatedpropellant charge, said charge being composed of a slow burning and fastburning propellants in an abutting R l s Igniting said slow burningpropellant in said confined zone opposite said formation to build up aresultant gas pressure sufficient to initiate a fracture in saidformation;

igniting said fast burning propellant in said zone by use of heatgenerated from the combustion of said slow burning propellant wherebythe gas pressure thus formed is adequate to extend said fracture a stillfurther distance from said well; and

thereafter removing said fluid from said formation via said well at anincreased flow.

5. in a process for stimulating the flow of fluid from an undergroundformation penetrated by a well, the improvement which comprises:

placing in a confined zone opposite said formation an elongatedpropellant charge, the latter being composed of a minor amount of a slowburning propellant at the lower end of said charge and the remainderbeing a fast burning propellant abutting against said slow burningpropellant, said fast burning propellant being composed of a pluralityof units abutting against one another;

first igniting and burning said flow burning propellant to producecombustion products in said zone in an amount and at a pressure adequateto initiate a fracture in said formation; and

igniting said fast burning propellant as the result of the combustion ofsaid slow burning propellant to produce a pressure within said zonesufficient to extend the thus newly formed fraction.

1. In a process for placing a fracture in an underground formationpenetrated by a well by the use of a generally cylindrical elongatedpropellant charge, the improvement which comprises: placing said chargein a confined zone opposite the portion of said formation to befractured, said charge having a separate minor amount of a slow burningpropellant and the remainder of said charge consisting essentially offast burning propellant, said slow and fast burning propellants being inan abutting relationship, igniting said slow burning propellant to buildup a resultant gas pressure sufficient to initiate a fracture in saidformation; and thereafter igniting said fast burning propellant in saidzone by use of heat generated from the combustion of said slow burningpropellant whereby the gas pressure thus formed is adequate to extendsaid fracture a still further distance from said well.
 2. The method ofclaim 1 wherein the slow and fast burning propellants are the samecomposition but the portion thereof constituting the fast burningmaterial has a substantially greater surface area per unit volume thanthe slow burning propellant.
 3. The method of claim 1 wherein said wellis a water injection well and wherein the formation in the immediatevicinity of said well and opposite said confined zone has decreasedsubstantially in its ability to take water at normal injectionpressures.
 4. In a process for stimulating the flow of fluid from anunderground formation penetrated by a well, the improvement whichcomprises: placing in a confined zone opposite said formation anelongated propellant charge, said charge being composed of a slowburning and fast burning propellants in an abutting relationship;igniting said slow burning propellant in said confined zone oppositesaid formation to build up a resultant gas pressure sufficient toinitiate a fracture in said formation; igniting said fast burningpropellant in said zone by use of heat generated from the combustion ofsaid slow burning propellant whereby the gas pressure thus formed isadequate to extend said fracture a still further distance from saidwell; and thereafter removing said fluid from said formation via saidwell at an increased flow.
 5. In a process for stimulating the flow offluid from an underground formation penetrated by a well, theimprovement which comprises: placing in a confined zone opposite saidformation an elongated propellant charge, the latter being composed of aminor amount of a slow burning propellant at the lower end of saidcharge and the remainder being a fast burning propellant abuttingagainst said slow burning propellant, said fast burning propellant beingcomposed of a plurality of units abutting against one another; firstigniting and burning said flow burning propellant to produce combustionproducts in said zone in an amount and at a pressure adequate toinitiate a fracture in said formation; and igniting said fast burningpropellant as the result of the combustion of said slow burningpropellant to produce a pressure within said zone sufficient to extendthe thus newly formed fraction.